Fig. 1. (a) Structural transition of the CsPbI3 material at different temperatures. Reproduced with permission [45], Copyright 2018, American Chemical Society. (b) Real part of dielectric function (ε1), absorption coefficient (α), and reflectivity (R) of MAPbI3 and CsPbI3 perovskites. Reproduced with permission [86], Copyright 2016, American Chemical Society. (c) Energy of the 1s transition (reflecting the evolution of the band gap) as a function of temperature for CsPbI3, CsPbI2Br, and CsPbBr3 perovskites. Reproduced with permission [87], Copyright 2017, American Chemical Society. (d) The calculated electronic band structures for the CsPbCl3, CsPbBr3, and CsPbI3 (cubic phase), including relativistic corrections, from density functional theory. Reproduced with permission [40], Copyright 2015, American Chemical Society. (e) Density of states of the cubic CsPbBr3 with corresponding contributions of elements to energy band. Reproduced with permission [88], Copyright 2016, Wiley-VCH. (f) Binding energy (R*), effective mass (μ), and dielectric constant (εeff) as a function of the band gap. Reproduced with permission [87], Copyright 2017, American Chemical Society. (g) Images and PL spectra of the perovskite colloidal solutions in toluene with different halide compositions. Reproduced with permission [40], Copyright 2015, American Chemical Society.

Fig. 2. (a) PL spectra of the MAPb(Br0.4I0.6)3 thin film over 45 s in 5 s increments. Reproduced with permission [48], Copyright 2015, RSC Publishing. (b) Time-dependent PL peak position in CsPb(BrxI1−x)3 of different halide compositions. Reproduced with permission [100], Copyright 2016, American Chemical Society. (c) PL spectra of CsPb(Br0.5I0.5)3 film with the illumination duration of 100 s. Reproduced with permission [52], Copyright 2017, Nature Publishing Group. (d) PL spectra of CsPb(Br0.1I0.9)3 with the illumination duration of 10 min. Reproduced with permission [50], Copyright 2017, American Chemical Society. PL spectra with the illumination of 5 min of the (e) CsPbI2Br and (f) CsPbBr2I films. Reproduced with permission [65], Copyright 2017, American Chemical Society. (g) PL spectra of the CsPb(BrxI1−x)3 with x ranging from 0.4 to 0.9. The solid lines were the spectra taken from freshly made samples, and the dashed lines were measured after 10 min illumination. Reproduced with permission [61], Copyright 2019, Nature Publishing Group.

Fig. 3. (a) Secondary electron (SE), cathodoluminescence (CL), and SE/CL overlay of the MAPb(I0.1Br0.9)3 film. The scale bar is 2 μm. Yellow-colored spots represent the signal from the I-rich clusters. Reproduced with permission [50], Copyright 2017, American Chemical Society. (b) Imaging the phase segregation in the CsPbIBr2 film: (i) secondary electron SEM image; (ii) CL mapping; (iii) and (iv) are the enlarged image of the highlighted area in (i) and (ii), respectively; (v) color-coded emission mapping of the film, where the orange regions have longer wavelength emission than the green region, indicating the accumulated iodine ions in the orange area. Reproduced with permission [58], Copyright 2017, Wiley-VCH.

Fig. 4. (a) PL spectra measured at 0, 2, 4, and 10 min for one position of the CsPbBr1.2I1.8 ensemble film excited at a laser power density of 30 W/cm² [62]. (b) PL spectra measured up to 50 min for one position of the single CsPbBr1.2I1.8 NC excited at a laser power density of 6 W/cm². Reproduced with permission [62], Copyright 2019, Nature Publishing Group. (c) PL spectra of CsPb(Br0.5I0.5)3 NCs film with the illumination duration of 120 s. Reproduced with permission [52], Copyright 2017, Nature Publishing Group. (d) Normalized PL spectra of the CsPb(BrxI1−x)3 NCs for 10 min. The dots represent the initial spectra while the lines represent the spectra after 10 min. Reproduced with permission [63], Copyright 2019, American Chemical Society.

Fig. 5. (a) Helmholtz free energy of the MAPb(BrxI1−x)3 perovskite as a function of the bromide concentration and temperature. Reproduced with permission [59], Copyright 2016, American Chemical Society. (b) Formation energy landscape of the CsPb(BrxI1−x)3 as a function of the bromide concentration. The configuration with a given x is illustrated by the grey dots and the lowest energy configuration is highlighted in red. Reproduced with permission [109], Copyright 2020, American Chemical Society. (c) Free energy of formation as a function of x in the MAPb(BrxI1−x)3 material. The blue lines represent the 0 K ground state formation energies. The green line is the free energies at 300 K. The red line represents the free energy after single photon absorption. Reproduced with permission [52], Copyright 2017, Nature Publishing Group. (d) Calculated Gibbs free energy of the CsPb(BrxI1−x)3 perovskite with (orange line) and without illumination (blue line). Reproduced with permission [61], Copyright 2017, Nature Publishing Group. (e) Photo-induced polaron trapping and associated energy scales related with light-induced phase segregation. Yellow spheres represent I ions, blue spheres represent Br ions, and pill shapes represent the MA cation. Reproduced with permission [50]. Copyright 2017, American Chemical Society. (f) Simulated time trace of the composition at the polaron for MAPb(I0.15Br0.85)3 and CsPb(I0.15Br0.85)3. Reproduced with permission [106], Copyright 2018, American Chemical Society. (g) Schematic illustration of the band diagram and carrier migration due to trap states in perovskite thin films. Evolution of the perovskite PL spectra upon illustration. Reproduced with permission [107], Copyright 2018, American Chemical Society.

Fig. 6. (a) Steady-state PL spectra of the CsPb0.75Sn0.25IBr2 perovskite with different illumination time. Reproduced with permission [105], Copyright 2018, Wiley-VCH. Normalized PL spectra of the CsPbBr0.37I0.63 bulk films with grain size of (b) up to 19.5 nm, (c) 46±7  nm, and (d) greater than 100 nm. Reproduced with permission [63], Copyright 2019, American Chemical Society. (e) The PL spectra of the CsPb(BrxI1−x)3 material embedded in the Cs4Pb(BrxI1−x)6 matrix with the illumination duration of 5 h. Reproduced with permission [61], Copyright 2019, Nature Publishing Group. (f) The PL spectra of the CsPb(BrxI1−x)3 microplatelet covered with PMMA layer with the continuous illumination for 12 min. Inset shows the spectra of the platelet without PMMA encapsulation after illuminated for 12 min. Reproduced with permission [120], Copyright 2019, RSC Publishing. (g) Normalized electroluminescence spectra of the red LED based on CsPb(BrxI1−x)3 NCs at a driving voltage of 5 V (g) with and (h) without KBr passivation. Reproduced with permission [115], Copyright 2020, American Chemical Society.